This invention concerns asphalt paving mix design and plant control methods, and more particularly, a method for determining and controlling the proportions of coarse aggregates, fine aggregates, and asphalt cement to be combined to produce optimum asphalt pavement mixtures in plant-mix type operations.
All asphalt pavement mix design is presently based upon knowledge and actual pavement experience of the contractor. It is purely a matter of judgement, thus resulting in a wide variety of standards and specifications.
Within the industry, there basically exists four types of asphalt road construction, Surface Treatment, Macadam Construction, Mixed-in-place Surfacing, and Plant Mix Type Construction.
The most advanced method of producing high quality pavements fall in the latter class of Plant Mix, to which this disclosure is related. In the Plant Mix Type method of pavement construction, a road bed and/or subgrade is prepared by grading, compacting and leveling to accommodate asphaltic pavement material to be placed thereon. The pavement mix is prepared at the site usually remote from the actual road surface and is carried to the road bed by truck prior to being leveled and compacted.
The primary purpose of a pavement is to transmit load from the surface to the subgrade or underlying soil. Because the pavement composite consists of aggregate generally varying in size from a mean diameter of 11/2 inches to fine power passing the number 200 screen mesh, one can appreciate the respective functions of the different sized aggregates in order to properly prepare a mixture. The larger aggregates in the mixture carry the load by coming in close proximity with one another and the sand or fine aggregates fill interstices between the larger aggregate. A mixture having too little fine aggregates and too many air voids is classified as an "open graded mix" and is very vunerable to deterioration due to water seepage and aeration, (aeration will shorten the asphalt cement life due to oxidation; this effects the viscosity and hardness of the asphalt). Conversely, poor stability results from an asphalt pavement mix having too little coarse aggregates (or too much fine aggregates) and will not produce a road with proper load bearing qualities.
In this case, the load is not transmitted through the rock, but instead through the plastic mixture of the finer particles and the asphalt binder thereby resulting in a more pliable, "mushy" pavement having a shorter life span with yielding qualities, and uneven riding surfaces.
There exists an optimum mixture of coarse aggregates, fine aggregates, and asphalt cement which will produce a pavement having the longest possible life and greater possible durability. The determination of such mixture is often ascertainable, but the actual production of such a mixture is the perplexing problem. This optimum combination of elements must provide a certain amount of air voids and void in the mineral aggregate in the mixture to allow for bitumen content. This affects durablity, flexibility, and stability of the mixture.
The development of the present state of the art has been hampered in part by firstly, non-uniform design and testing standards that do not conform to specifications and acceptance standards, and secondly, the lack or understanding that only the quality (gradation, shape and absorption) of the fine aggregate, not that of the entire mixture of aggregates, is the only variable appreciably effecting the void volume. Appreciation of surface area of aggregate in determining the best proportions of asphalt to mineral aggregate and the importance of pore size on the permeability of the bituminous mixture is discussed in "The Movers Asphalt Pavement" by Clifford Richardson, New York, 1913. Engineers presently established a Job Mix Formula based on investigation and testing of only one gradation, thus not appreciating the range of possibilities in relation to the entire gradation spectrum of aggregates.
In addition to the problem of determining the quantity of various elements to be mixed, there exists other quality control problems that come about during the plant manufacturing and lay down process. Of particular importance is the task of maintaining a constant Job Mix Formula in view of the segregation and degradation of aggregates which occurs during crushing, storage, mixing, tumbling, and transporting and lay down operations.
Another problem often encountered in determining the exact amount of asphalt cement to be added to the mixture results from varying absorption characteristics of the mineral aggregate. Certain types of mineral aggregates absorb more or less asphalt cement, thereby causing the effective volume of asphalt cement in the mixture to vary accordingly, which ultimately affects the volume of voids filled by the bitumen. For example, should the aggregate used be visicular or permeable in nature, relatively more bitumen (asphalt cement) would be absorbed within the cracks, holes and crevices located therein. This absorption substantially affects the effective volume of asphalt cement available to fill the voids.
The amount of effective asphalt cement, which ultimately effects the amount of air voids, utilized within the asphaltic mixture must be closely controlled to produce desired results. Void volume control in the mixture is critical. Insignificant variations in the effective proportion of asphalt cement resulting from variations in aggregate absorption characteristics may substantially affect the life and quality of the pavement. Prior art methods for void control involve manipulations of the Job Mix FormUla over the entire gradation scale. Specific reference is made to Fuller maximum density curves and the Federal Highway Administration 0.45 Power Gradation Chart shown in "Mix Design Methods for Asphaltic Concrete and other Hot-Mix Type" published by The Asphalt Institute, manual series no. 2, fourth edition, March, 1974. The basis for the FHA 0.45 power gradation chart is described in detail in volume 31, pages 176 through 207 of the "Proceedings of the Association of Asphalt Paving Technologist", Jan. 29, 1962.
Additionally, because aggregate from different rock quarries and pit locations have varying absorptive qualities, samples taken during the production process must be constantly monitored and analyzed in order to produce an even uniform pavement having consistent riding and frictional qualities. The present state of the art does not provide a viable method to obtain the desired analysis of production samples within a sufficient time period to make the necessary corrections at the plant site to compensate for such errors during production.
Due to the inability to precisely control the gradation as heretofore mentioned, contractors are constantly faced with the problem of meeting the stringent contractural Job Mix Formula specifications. A typical analysis of a sample mix may take days or weeks to determine whether the plant is functioning correctly. During that period of time, disastrous results in voidage control may have occurred during the lay down process thereby resulting in improper mix proportions, too high or too low amounts of asphalt cement, or the laying of open graded mixtures.
The premises considered, the special problems associated with conventional methods of construction of plant mix asphalt pavements may be summarized as follows:
(1) The asphalt tolerance is very critical and difficult to maintain during production due to the variable quality of aggregates;
(2) Uniformity of the mixture of mineral aggregates as represented by the Job Mix Formula is difficult to maintain due to degradation and segregation of aggregates during stockpiling, mixing, and transporting; and
(3) Precise control of the air voids within the compacted mixture is difficult.
(4) Plant control to achieve the optimum combination of coarse and fine aggregates to place within the mixture is difficult to ascertain;